Systems for controlling protein interactions have a variety of experimental and commercial applications. The majority of such systems that have been developed are based on the administration of chemical effectors which require diffusion into the cell to provide appropriate, non-toxic concentrations of chemical at the dimer-interface site. In an alternative strategy, attempts have been made to use light to regulate protein interactions. See, e.g., Leug et al., PNAS, Sep. 2, 2008, vol. 105, no. 35, 12797-12802; Shimizu-Sato et al., Nature Biotech. Vol. 20, page 1041-44 (2002); Tyszkiewicz et al., Nature Methods, Vol. 5, No. 4 (2008), p. 303-305.
The phytochromes comprise a family of biliprotein photoreceptors that enable plants to adapt to their prevailing light environment (Kendrick and Kronenberg (1994) Kendrick, Pp. 828 in Photomorphogenesis in Plants, Dordrecht, The Netherlands: Kluwer Academic Publishers). Phytochromes possess the ability to efficiently photointerconvert between red light absorbing Pr and far red light absorbing Pfr forms, a property conferred by covalent association of a linear tetrapyrrole (bilin or phytobilin) with a large apoprotein. Phytochromes from cyanobacteria, to green algae and higher plants consist of a well conserved N-terminal polypeptide, roughly 390-600 amino acids in length (see, e.g. U.S. Pat. No. 6,046,014), to which the phytobilin prosthetic group, e.g., phytochromobilin (PΦB) or phycocyanobilin (PCB) is bound.